Proposed maintenance hipot levels for generator rotor windings

Hipot testing is always used as a factory or commissioning test to ensure that the groundwall insulation in new stator windings and wound rotor windings is fit for service. Maintenance hipot testing is also used by some motor and generator owners to periodically ensure that the insulation in windings that have been in operation will not fail when returned to service. The dc and ac hipot test levels for factory or commissioning tests are well established by IEEE/ANSI and IEC. Several standards also provide guidance on maintenance hipot tests for stator windings. However, there is no such guidance given for maintenance hipot tests of rotor windings. This paper reviews the standards applicable for generator rotor winding hipot testing, and highlights the lack of any guidelines by IEEE on test levels for generator rotors that have seen service. Since an important purpose of maintenance hipot testing is to ensure that the insulation can withstand voltage transients that can occur in operation, a review is made of the transient voltage stresses that the rotor ground insulation may see. Based on the transients that are possible, it is proposed that the rotor winding maintenance ac hipot test level for large machines be set to a minimum of 1000 Vac, rms, or half the final acceptance hipot (whichever is higher).